Linear machine having a primary part and a secondary part

ABSTRACT

A linear machine comprises a primary part and a secondary part. The primary part forms a receptacle around an axis and has a plurality of annular primary coils, which are arranged concentrically with respect to the axis, can have alternating current applied to them, and are separated by intermediate elements, in order to produce a magnetic field in the receptacle. The secondary part, which can be moved relative to the primary part by the magnetic field in the receptacle along the axis, is provided with secondary coils having superconductor windings. In order to produce a linear motor which allows high force densities, the intermediate elements are produced from non-magnetizable material and the primary coils and the secondary coils are arranged with an air-gap winding, wherein the secondary coils are manufactured from a high-temperature superconductor and direct current can be applied or is applied to them. Force densities of more than 18 N/cm 2  can be achieved in the receptacle by the linear motors.

This application claims priority to and the benefit of the filing date of International Application No. PCT/EP2008/002333, filed 25 Mar. 2008, which application claims priority to and the benefit of the filing date of German Application No. 10 2007 015168.5, filed 27 Mar. 2007, both of which are hereby incorporated by reference into the specification of this application.

The invention relates to a linear machine having a primary part, which has a plurality of annular primary coils, which are arranged concentrically with respect to an axis and are separated from one another by intermediate elements, and having a secondary part which has a plurality of secondary coils, to which direct current can be applied and which are arranged axially alongside one another with alternating polarity and have superconductor windings, with one part being movable backward and forward relative to the other part parallel to the axis.

BACKGROUND

DE 195 42 551 A1 discloses a linear motor having a hollow-cylindrical primary part, which has annular primary coils which are arranged concentrically with respect to a movement axis of a secondary part and can be operated with polyphase current. Annular laminates composed of soft-magnetic material are arranged between the primary coils, are used as intermediate elements to separate adjacent primary coils, and form magnetizable teeth, in order to amplify the magnetic flux and to pass this to the receptacle in which the secondary part is arranged. The primary coils and the annular laminates are accommodated in a hollow-cylindrical yoke composed of magnetizable material, which forms a magnetic return path. The secondary part is arranged such that it can move axially within the receptacle that is formed by the primary part. The secondary part has a plurality of field magnets composed of superconductor windings, which are arranged one behind the other with alternating polarity in the axial direction. In DE 195 42 551, the magnetic fields of the secondary windings are at right angles to the axis of the secondary part. In order to produce this field direction using wound coils, the axis of each individual coil through which current flows must be at right angles to the movement axis of the linear motor. Only if permanent magnets or superconducting solid-body magnets are used can these magnets rest with their inner circumferential surface on a cylindrical yoke composed of magnetizable material. Although these then have an annular shape, they are magnetized radially, however. In the case of wound secondary coils, in contrast, an arrangement must be chosen in which the wound coils are offset alongside one another in the circumferential direction and in the axial direction on the casing surface of the supporting body. The magnetic forces which are produced when current is applied to the primary and secondary coils produce a relative movement between the primary part and the secondary part.

EP 1 465 328 A1 discloses a linear motor in which the primary part and secondary part are arranged reversed, such that the secondary part is on the outside, and surrounds the primary part.

The capability to magnetize the soft-magnetic teeth is restricted because magnetic saturation occurs in the soft-magnetic material. In order to achieve higher force densities between the primary part and secondary part with high current densities in the coils of the primary part, it has been proposed that the number of turns in the primary coils be increased or that the amount of magnetizable material be increased. These measures have allowed force densities of about 8 N/cm² to be achieved in the trial stage for round and polysolenoid linear motors. However, the physical size and the weight of the linear motors have to be significantly increased to do this.

A concept for a linear motor, in which the stator has primary coils composed of a superconductor material which comprise high-temperature superconducting double-pancake coils, is known from Superconductor Science and Technology, 17 (2004), page 445 to 449. In order to achieve a force density in the order of magnitude of 14 N/cm² with the linear motor, an actuator is proposed which is fitted with NdFeB magnets. As an alternative concept, an actuator is proposed which comprises solid-body superconductors, which are formed by means of a combination of iron laminate wafers and YBCO.

EP 0 425 314 A1 discloses a linear motor in which both the coils in the primary part and the coils in the secondary part comprise saddle-type coils which are curved in the form of an arc and are arranged axially offset with respect to one another. The magnetic field of the saddle-type coils in the primary part and in the secondary part is at right angles to the movement axis.

SUMMARY OF INVENTION

In accordance with the present invention, provided is a linear machine in which considerably higher force densities are made possible by design measures on the primary part and/or secondary part, even for linear machines of small physical size.

According to one aspect of the invention, the arrangement of the primary coils in the primary part is in the form of an air-gap winding with intermediate elements composed of non-magnetizable material, and the secondary coils are composed of windings of a high-temperature superconductor, as a result of which force densities of more than 18 N/cm² can be achieved. The linear machine is in the form of a linear motor, in which a relative movement is produced between the primary part and secondary part parallel to the axis by applying current to the primary and secondary coils, via the magnetic fields that are produced in this way, and the invention will be described in the following text primarily with reference to this. However, the linear machine may also be in the form of a generator, in which a current which is induced in the primary coils by the relative movement between the primary part and secondary part is converted in order to obtain energy. The high force densities when the linear machine is in the form of a linear motor can be achieved by applying alternating current to the primary coils and direct current to the secondary coils. Since the arrangement of the primary coils and also the arrangement of the secondary coils are in the form of an air-gap winding, that is to say no magnetizable material for flux guidance is arranged either between the primary coils or between the secondary coils, the force density in the case of the linear machine according to the invention is not limited by saturation magnetization.

According to another aspect, current level in the primary part, that is to say the current in the circumferential direction per axial length of the primary part, can be increased in comparison to known linear motors without enlarging the physical size of the linear motor, as a result of which the force density, which is proportional to the current level, rises without saturation effects. No iron or magnetizable material for concentration of the magnetic flux is arranged between the primary coils. The use of secondary coils composed of high-temperature superconducting material, which has a critical temperature which is higher than 77 K, in the secondary part allows large direct currents to be applied to the secondary coils, in order to make it possible to produce extremely strong magnetic fields in the receptacle. A further advantage with the linear motor according to the invention is that a force profile which is virtually smooth in the axial direction is achieved since the air-gap winding means that there are largely no reluctance forces in practice, and in consequence scarcely any cogging forces occur. Furthermore, since there are no permanent magnets and magnetizable material in the primary part and secondary part, and no magnetic forces therefore occur when the current that is supplied is switched off, the linear motor can be serviced and cleaned relatively easily.

According to yet another aspect, a high current level in the primary part can be achieved in particular by choosing a high filling factor for the primary part. The filling factor is defined as the volume ratio of the volume of the primary coils through which current flows to the volume of the intermediate elements and any intermediate spaces that there may be between the primary coils. The filling factor of the primary part is greater than 70%, and in particular greater than 85%. Primary coils which are adjacent in the axial direction preferably have an alternating current with a phase shift of 120° applied to them, as a result of which the linear motor forms a three-phase motor. In the case of a two-phase motor or a polyphase motor with more than three phases, the phase shift may be adapted or chosen differently.

According to one exemplary embodiment, the primary coils may have windings composed of a normal conductor, in particular such as a conductor composed of aluminum or copper, as a result of which the primary coils may, for example, be liquid-cooled or gas-cooled in a cost-effective manner. Cooling with water or oil, for example, is particularly advantageous. In particular, the normal conductor may also be formed from a hollow conductor, whose internal tube is used for cooling. Alternatively, according to another exemplary embodiment, the windings of the primary coils could be composed of or be manufactured from a superconducting conductor, in particular a high-temperature superconducting conductor. The current that is applied should then be applied using alternating current at a frequency of less than 100 Hz, in particular of less than 50 Hz, in order to keep alternating-current losses in the superconducting primary coils low, which would otherwise have to be compensated for by additional coolant. In the linear motor according to the invention, force densities of more than 18 N/cm² can be achieved, and when using superconductors both in the secondary coils and in the primary coils, it is even possible to achieve force densities of more than 25 N/cm². Cooling lines through which a coolant can flow may also be formed between the coils, or gaps may be left open between the primary coils and if appropriate the intermediate elements, in order to cool the primary coils. The intermediate elements may be in the form of annular segments thus allowing a coolant to be passed to the end faces, which are not covered by the annular segments, of the primary coils. The intermediate elements may extend over the entire area, partially or with intermediate spaces over the radial height of the primary coils. The intermediate elements may also comprise grid structures, hollow bodies or grid bodies, which are sufficiently mechanically robust and at the same time allow a coolant to flow through them.

Furthermore, according to yet another aspect, the primary coils and the intermediate elements are sheathed by a yoke, which is composed of non-magnetizable material, in particular a lightweight material without any iron. Alternatively, the yoke may be composed of material which does contain iron and/or which can be magnetized, for magnetic field shielding. In particular, the yoke and the intermediate elements may form a mechanical holding structure for the primary coils. In order to anchor the intermediate elements in the axial direction as well, the yoke may have slots on its internal circumference, in which slots the intermediate elements engage in an interlocking manner. Anchoring the intermediate elements on the yoke allows the primary coils to be supported in the axial direction on the intermediate elements, which means that the yoke can absorb the magnetic field forces which act on the primary coils, in the axial direction. According to one exemplary embodiment, forming the primary part without any iron can achieve a particularly lightweight design for the primary part and therefore for the linear machine, while avoiding saturation effects at the same time. Alternatively, the yoke may have a magnetizable material in order to form a return path for the magnetic flux.

According to still yet another aspect, the primary coils may be encapsulated in plastic, for example in synthetic resin, in particular in epoxy resin. The intermediate elements in one advantageous refinement of the invention are likewise manufactured from plastic, for example synthetic resin, in particular epoxy resin, and can be reinforced with fiber reinforcement, for example by insertion of glass fiber material.

According to still yet another aspect, the superconducting secondary coils can carry high current densities, for example current densities of more than 50 A/mm², furthermore of more than 70 A/mm² and in particular more than 100 A/mm², thus making it possible to produce an extremely strong magnetic field by means of the secondary coils. The flux densities which can be produced by the secondary part in the air-gap may reach more than 0.5 Tesla, more than 1 Tesla, and possibly up to 2 Tesla. The secondary part has a cylindrical supporting body adjacent to or on whose casing surface the secondary coils are arranged. The supporting body of the secondary part is produced from a non-magnetic material, for example from fiber-reinforced plastic. The supporting body could also be produced from or be composed of a magnetic material, in particular iron. In one refinement, the secondary coils have an annular shape and are arranged concentrically with respect to one another with respect to the axis, mounted on the associated supporting body of the secondary part. Secondary coils which are adjacent in the axial direction have direct current applied to them in antiphase, by opposite polarity connection, during operation. Once again, in order to create the air-gap winding, non-magnetizable, annular spacing elements can be arranged between the secondary coils, on which spacing elements the secondary coils are supported in the axial direction. In this exemplary embodiment, adjacent secondary coils are at a distance from one another which is at least twice as great, and preferably more than this, as the width of the respective secondary coils in the axial direction. A plurality of coils can also be combined to form a pack, all having the same current flow direction (connected in series or in parallel). A reverse current direction is then in each case applied only to adjacent coil packs.

Further advantages and features of the invention will be described with reference to exemplary embodiments, which are illustrated schematically in the drawing, of a linear motor as a linear machine.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a linear motor according to the invention with a primary part and a secondary part, according to a first exemplary embodiment and in the form of a longitudinal section; and

FIG. 2 shows a perspective view of the secondary part from FIG. 1.

DETAILED DESCRIPTION

Referring now to the drawings wherein the showings are for the purpose of illustrating preferred and alternative embodiments of the invention only and not for the purpose of limiting same, FIG. 1 illustrates a linear motor, which is annotated 10 in its totality, with a primary part 20 and a secondary part 30. The primary part 20 bounds a cylindrical receptacle 11 in which the secondary part 30 can move backward and forward along a central axis A. In the illustrated exemplary embodiment, the primary part 20 has five primary coils 21 which are arranged concentrically with respect to the axis A. The drawing shows only one motor section from an entire motor since, for example, the number of coils or coil packs must be divisible by three, for example, for three-phase operation. The primary coils 21 comprise annular disk coils to which phase-shifted alternating current or three-phase current can be applied, phase-shifted through 120°, for example, on the external circumference via contacts which are not illustrated, in order to produce a magnetic traveling field by means of the primary coils 21 in the receptacle 11. The windings, which are composed of a copper conductor, of the primary coils 21 are encapsulated in epoxy resin, to provide mechanical robustness. Annular intermediate elements 22 are likewise arranged between the primary coils 21, on which intermediate elements 22 the end faces of the primary coils 21 are supported in the axial direction. The intermediate elements 22 extend in the axial direction from the internal circumference of the primary coils 21 to the external circumference of the primary coils 21. A hollow-cylindrical yoke 23, on which the intermediate elements 22 are anchored (not illustrated), rests on the external circumference of the intermediate elements 22 and of the primary coils 21. The yoke 23 and the intermediate elements 22 thus form a mechanical holding structure for the primary coils 21 that are accommodated therein.

The yoke 23 around the primary part 20 may be composed of non-magnetizable material or, for shielding purposes, also of magnetizable material. In the latter case, it is even possible for the force density to be increased. If the yoke 23 is composed of electrically conductive material, then it can preferably be formed by means of laminated or slotted materials, in order to reduce alternating-current losses.

By way of example, the intermediate elements 22 may be composed of glass-fiber-reinforced plastic and, according to the invention, therefore cannot be magnetized, as a result of which the magnetic field which is produced in the receptacle 11 when current is applied to the primary coils 21 is not limited by saturation magnetization of the intermediate elements 22. There is essentially no magnetizable material for flux guidance located between the primary coils 21. The arrangement of the primary coils 21 located alongside one another in the axial direction is therefore in the form of a so-called air-gap winding. These “air-gaps” between the primary coils 21 are filled with the intermediate elements 22, which are possibly partially hollow and/or are used exclusively for insulation. Very broad primary coils 21 with a large number of turns per unit axial length can therefore be used in the primary part 20. Since the volume of the intermediate elements 22 occupies only a fraction of the volume of the primary coils 21, the filling factor of the primary part with turns which carry current and also produce a magnetic field (traveling field) is considerably more than 50%. A higher current can therefore be introduced into the primary coils 21 of the primary part 20.

The secondary part 30, which is illustrated in FIG. 1 and FIG. 2, has annular secondary coils 31, which are arranged concentrically with respect to the axis A and are composed of a high-temperature superconductor. These secondary coils 31, which are superconductive at cryogenic temperatures of more than 20 K, have direct current applied to them, with secondary coils 31 which are adjacent in the axial direction being connected in antiphase. The high-temperature superconductor windings and secondary coils 31 in the secondary part 30 may be in the form of pancake coils, double-pancake coils, packs composed of these pancake coils or short solenoid coils. Annular spacing elements 32 are likewise arranged between the secondary coils 31, and are arranged concentrically with respect to the axis A. The spacing elements 32 are composed of glass-fiber-reinforced epoxy resin and are arranged together with the secondary coils 31 on a hollow-cylindrical supporting tube 33. The hollow-cylindrical supporting tube 33 may be manufactured from soft-magnetic magnetizable material such as soft-magnetic iron, or may likewise be composed, for example, of glass-fiber-reinforced plastic. In order to allow the secondary coils 31 to be cooled, for example using liquid nitrogen, the cryostat 34 is provided with a double-walled tube 36. The intermediate space, which is not illustrated, between the “warm” outer tube wall and the “cold” inner tube wall of the tube 36 is evacuated, in order to prevent heat from being introduced from the outside into the cryostat 34, or to constrain it. If required, an insulation layer composed of commercially available super insulation sheet can also be fitted around the cold tube wall. Force is transmitted from the secondary part 30 to the cryostat 34 by means of schematically indicated transmission elements 35 a and 35 b. The transmission elements 35 a, 35 b are composed of a material of low thermal conductivity and high mechanical strength, for example of glass-fiber-reinforced plastics. The secondary coils 31 can be operated with current densities of up to 100 A/mm². The linear motor 10 with a primary part 20 designed according to the invention and with an air-gap winding of the primary coils and a secondary part 30 designed according to the invention allows force densities of more than 18 N/cm² to be achieved in the receptacle 11 between the primary part and the secondary part, in order to move the secondary part 30 parallel to the axis A.

Numerous modifications will be evident to a person skilled in the art from the above description and the dependent claims. The number of primary and secondary coils in the axial direction is only an example and, in particular, may vary with the width of the coils and the overall length of the linear motor. The secondary coils may also be arranged in a spiral shape. The yoke and the supporting tube for the secondary part may also be composed of material containing iron. The supporting tube for the secondary part may also be omitted, if the secondary coils have been firmly connected to one another together with the spacers, for example by vacuum impregnation. Alternatively, the supporting tube for the secondary part may be composed of laminated and slotted magnetizable material, or likewise, for example, from glass-fiber-reinforced plastic. Hard-magnetic materials may also be used as a supporting tube in the secondary part through which direct current flows. Particularly when using normally conductive primary coils, they can be cooled indirectly or preferably directly, for example by water, oil, gas or nitrogen (N₂). Alternatively, it is also possible to use suitable gas or dry cooling, which allows an operating temperature of below 77 K, for example 20 K or 30 K. In order to further reduce eddy current losses in the primary part, the primary coils may be provided with braided-wire windings. If required, a second primary part could also be arranged within the secondary part, in order to further increase the force density. Instead of the secondary part, the primary part could also be moved parallel to the axis by the magnetic field that is produced when current is applied. The primary part could be arranged internally, and the secondary part could be arranged externally. In one refinement of the linear machine as a generator, the secondary part to which direct current is applied could be moved mechanically, for example by a rising and falling buoy of a wave-driven power station. The current which is induced in the primary windings of the primary part by means of this movement of the secondary part could be used to obtain energy, with the linear machine then acting as a generator. Instead of the secondary part, the primary part could also carry out the backward and forward movement parallel to the axis with the secondary part being stationary, without departing from the scope of protection of the attached claims.

It will be appreciated that various of the above-disclosed and other features and functions, or alternatives thereof, may be desirably combined into many other different systems or applications. Also that various presently unforeseen or unanticipated alternatives, modifications, variations or improvements therein may be subsequently made by those skilled in the art which are also intended to be encompassed by the following claims. 

1. A linear machine comprising: a primary part having a plurality of annular primary coils, arranged concentrically with respect to an axis (A) and separated from one another by intermediate elements, and a secondary part having a plurality of secondary coils aplliable with direct current and arranged axially alongside one another with alternating polarity and having superconductor windings, with one part being movable relative to the other part parallel to the axis, wherein the arrangement of the primary coils in the primary part is in the form of an air-gap winding with the intermediate elements composed of non-magnetizable material, and the secondary coils are composed of windings of a high-temperature superconductor, as a result of which force densities of more than 18 N/cm² can be achieved, with the secondary coils being annular and being arranged concentrically with respect to one another around a supporting body, and with spacing elements being arranged between the secondary coils, on which spacing elements the secondary coils are supported in the axial direction.
 2. The linear machine as claimed in claim 1, wherein the arrangement of the secondary coils in the secondary part is in the form of an air-gap winding.
 3. The linear machine as claimed in claim 1, wherein no magnetizable material for concentration of the magnetic flux is arranged between the primary coils of the primary part and between the secondary coils of the secondary part.
 4. The linear machine as claimed in claim 1, wherein a volume ratio of the primary coils to the intermediate elements is more than 70%.
 5. The linear machine as claimed in claim 1, wherein the primary coils are composed of windings of a normal conductor composed of one of aluminum and copper.
 6. The linear machine as claimed in claim 1, wherein the primary coils are manufactured from windings of a superconductor.
 7. The linear machine as claimed in claim 1, further including a yoke which sheaths the primary coils and the intermediate elements and is composed of one of non-magnetic material and non-magnetizable material.
 8. The linear machine as claimed in claim 7, wherein the yoke has slots on its internal circumference, on which slots the intermediate elements are anchored.
 9. The linear machine as claimed in claim 1, wherein one of the primary coils (21) and the secondary coils are encapsulated in a sheath, with the intermediate elements at least partially comprising the sheath.
 10. The linear machine as claimed in claim 1, wherein a current density of more than 50 A/mm² is applied to the secondary coils, and the magnetic field of the secondary coils is aligned parallel to the axis.
 11. The linear machine as claimed in claim 1, wherein the supporting body is cylindrical and the secondary coils are arranged about an outer surface of the supporting body.
 12. The linear machine as claimed in claim 11, wherein the supporting body is one of non-magnetizable and composed of non-magnetizable material.
 13. The linear machine as claimed in claim 1, wherein the spacing elements are one of non-magnetizable and composed of non-magnetizable material.
 14. The linear machine as claimed in claim 13, wherein the secondary coils have a width, and the distance between adjacent secondary coils corresponds at least to twice the width of the secondary coils.
 15. The linear machine as claimed in claim 1, wherein alternating current is applied to the primary coils, the primary part and secondary part being moved relative to one another by applying current to the primary and secondary coils, and the linear machine forms a linear motor.
 16. The linear machine as claimed in claim 15, wherein the primary coils are manufactured from windings of a superconductor, with the alternating current being applied oscillating at a frequency of less than 100 Hz.
 17. The linear machine as claimed in claim 1, wherein one of the primary part and the secondary part can be moved parallel to the axis, on an externally-operating basis, wherein current which is induced in the primary coils by the axial movement between the primary part and the secondary part can be tapped off, and the linear machine forms a generator.
 18. A linear machine including a primary part and a secondary part, the primary part of the linear machine comprising: a plurality of annular primary coils which are arranged concentrically with respect to an axis and are separated by intermediate elements, the arrangement of the primary coils in the primary part is in the form of an air-gap winding with intermediate elements composed of non-magnetizable material, and alternating current applied with a phase shift to the primary coils which are located alongside one another.
 19. A linear machine including a primary part and a secondary part, the secondary part of the linear machine comprising: a plurality of secondary coils which are arranged axially alongside one another and have superconductor windings, the arrangement of the secondary coils is in the form of an air-gap winding, wherein the secondary coils are manufactured from a high-temperature superconductor and direct current of opposite polarity is applied to secondary coils which are located alongside one another. 